If your in the US and have a store called "Five below" you can get figures for $5 each new from giants that will work on all games (except SSA if they werent in it), you can also get the wii u copy of the game for $5 (yes no joke, great if u own a wii u)
Gamestop is doing buy 2 get 3 on used like Sleepy0429 said, blue based versions are prob best to get as the series 2/3/4 have the path resets on skills that series 1's lack. If you dont care about that, any figure will pretty much work as long as its on that games roster.
I cant comment on SSA as ive yet to play it (ya i know I need to) because I started when giants hit but I hear great things. Each game is great in its own way, Alot of stores have discount going on the start packs if you look around.
Oh and if you do start with trap team or supercharges you can use those portals on the presious games so u could buy them all used as long as u have a trap team or supercharger portal. (im assuming here since trap teams works with supercharges and there is a portal owners digi pack that traps portal works on superchargers etc)
Also if you have a wii and want SSA new, walmart has it in stock it seems
http://www.walmart.com/ip/46179342?reviews_limit=9&